Colorless Polyamide–Imide Films with Enhanced Thermal and Dimensional Stability and Their Application in Flexible OLED Devices

In this study, we intend to solve the problem of the low glass transition temperature (Tg) and the high coefficient of thermal expansion (CTE) that colorless polyimide (PI) films always exhibit. First, we copolymerized 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine (TFDB) with 4,4′-(hexafluoroisopropene) diphthalic anhydride (6FDA) and p-phthaloyl chloride (TPC) and obtained a series of transparent and colorless films (PAI-10). These PAI films showed a high Tg of 346–362 °C and a low CTE of 25.2–62.9 ppm/K, which were better than those of PI6FDA-TFDB. Subsequently, three rigid diamines, p-phenylenediamine (p-PDA), 5-amino-2-(4-aminophenyl)benzoxazole (DAPBO) and 5-amino-2-(4-aminophenyl)benzimidazole (DAPBI), were used to replace 10% of the TFDB (PAI-9P-5, PAI-9O-5, and PAI-9I-5, respectively) to investigate the effect of the rigid structure on the thermal and optical properties. The results showed that rigid structures can greatly improve the thermal properties but impair transparency to some extent. Among these, PAI-9O-5 possessed the best performance with T430 of 81.9%, Tg of 364 °C, and CTE of 19.8 ppm/K. Furthermore, we fabricated an organic light-emitting diode (OLED) with PAI-9O-5 as the substrate. The flexible and bendable OLED@PAI-9O-5 showed typical green emission at 520 nm, a turn-on voltage of 2.9 V, a luminance of 1000 cd/m2 at 5 V, a current efficiency (CE) of 2.53 cd/A, and an external quantum efficiency (EQE) of 0.81%, which were comparable to those of devices fabricated on conventional glass substrates.